1. Technical Field
The disclosure relates in general to a trim circuit for a power supply controller and more particularly to a trim circuit having a capacitance type programmable circuit connection.
2. Background
Electronic devices use power to operate. During conversion, a low frequency (e.g. 50 Hz or 60 Hz) programming voltage alternating current (AC) is converted to programming voltage direct current (DC); the programming voltage DC is converted to high frequency (e.g. 30 to 300 kHz) AC; this high frequency programming voltage AC is transformed (by for example a transformer) to a lower voltage to provide safety isolation. The output of the transformer is rectified to provide a regulated DC output, which may be used to power an electronic device.
In order to compensate process variations, analog integrated circuits such as power supply controllers in the power supply may be commonly trimmed for critical parameters. Trim may be used for wafer sorting (i.e. wafer leveling) or at IC level.
During wafer sorting, the power supply controller may be trimmed using trim pads on the wafer before being assembled in plastic packages. Trimming is done at the wafer level because the trim pads are not usually accessible after assembly. Besides, the assembled parts may be tested again at final test (FT) to eliminate parts shifted too much or damaged during assembly.
Programmable fuses and antifuses allow IC designers to “personalize,” or custom configure, various circuits. A programmable fuse is in a close circuit connection if unprogrammed and in an open circuit connection if programmed. On the contrary, a programmable antifuse is in an open circuit connection if unprogrammed and in a close circuit connection if programmed. Generally, a programmable fuse comprises a fusible conductive link that is broken during programming so that the conductive link no longer closes the circuit. Programmable fuses are a laser programmable type or an electronically programmable type. In both types, the fusible conductive link is broken by heating the link sufficiently so as to melt the link. In the laser programmable type of fuse, a laser provides energy to melt the conductive link. In the electronically programmable type, a relatively large current is passed through the conductive link such that the resistive heating of the link causes the link to melt.
FIG. 1A shows a prior trim circuit. As shown in FIG. 1A, the sensing circuit 110 senses the voltage at node N1 to output a logic signal SL. The voltage of the node N1 is corresponding to the state of the fuse 120. In initial fabricated state (i.e. normal state), the fuse 120 provides a low resistance connection or short circuit connection, so that the logic signal from the sensing circuit 110 is logic “0”. After the fuse 120 is programmed, the fuse 120 provides a high resistance or open circuit connection, so the logic signal from the sensing circuit 110 turns to logic “1”.
However, in prior art, in order to program the fuse 120 which may be metal or poly, a large current is required. FIG. 1B shows the current-voltage (I-V) curve for the prior fuse 120. As shown in FIG. 1B, in order to program the metal or poly fuse 120, the current must be large enough, for example 400˜800 mA. This large current is provided by an external and large-size current source.
Further, in the prior trim circuit in FIG. 1A, in order to program/trim, each trim bit needs two trim pads 130A and 130B. The trim pad is large size, for example, 50 μm×50 μm. Further, an external large-size current source is also required to provide a large program current. Therefore, in the prior art, the trim circuit is large sized due to the large-size pads and the external large-size current source.